I. Field of the Invention
The present invention relates generally to a method for producing a flat hem with a very sharp radius bend between two sheet metal panels for use primarily as automotive closure.
II. Description of Related Art
There are many previously known hemming machines and hemming methods. Many industries, such as the automotive industry, utilize sheet metal hemming machines to secure two metal panels together. For example, in constructing a door for an automotive vehicle, the door typically comprises both an outer panel and an inner panel. In order to secure these panels together, a hem is formed between the inner and outer panel around the outer peripheral edge of the panels such that an outer edge portion of the inner panel is sandwiched in between a flange on the outer panel and the outer panel itself.
In order to perform the hemming operation, there are many previously known hemming machines. These hemming machines typically comprise a base and hemming tooling mounted to the base. A nest is also mounted to the base and the nest and hemming tooling are movable relative to each other. The nest, in turn, supports the panel assembly to be hemmed.
In order to form the hem, a flange is first formed around the outer periphery of the outer panel prior to the hemming operation. This flange, furthermore, lies in a plane that is generally perpendicular or with an angle of 80 degrees to 120 degrees to the plane of the outer panel. Typically, the flange has a width of approximately 6 to 12 mm.
After the flange is formed in the outer panel by a separate flanging operation, the outer panel is then positioned on the nest and the inner panel positioned upon the outer panel so that an outer edge of the inner panel is spaced slightly inwardly from the bend line between the outer panel and its flange. Thereafter, the flange is compressed first against a prehemming tool which bends the flange approximately 45 degrees relative to the plane of the outer panel and so that the flange overlies the outer peripheral portion of the inner panel. The now bent flange is then compressed against the final hemming tool thus sandwiching the outer peripheral portion of the inner panel in between the flange and the outer panel thereby completing the panel assembly.
In order to improve the visual appearance of the hem, many industries, and particularly the automotive industry, have increasingly demanded that the overall hem be as thin as possible. This, in turn, creates a visual optical illusion of decreasing the gap space between the hem and the adjacent panel on the vehicle. Minimization of this apparent gap space between adjacent panels is highly desirable.
Special problems, however, have arisen when hemming the inner and outer panels that are constructed from aluminum sheet metal. As shown in FIG. 1, in these previously known hemming methods, the flange 100 is first formed on the aluminum sheet metal panel 102 so that the outer radius of the bend line 104 between the flange 100 and the remainder of the outer panel 102 is formed at a radius R of approximately 1.2 mm+t where t=the thickness of the aluminum panel. The subsequent hemming operation on such aluminum panels, i.e. compressing the flange initially against the prehemming tooling and subsequently against the final hemming tooling, has created several distinct problems which have previously been unsolved.
With reference to FIG. 2, first, by forming the flange with a relatively large radius, i.e. 1.2 mm plus the thickness of the panel 102, compression of the flange 100 against a conventional 45 degrees prehemming tooling 106 causes the bend line 104 to creep inwardly from the position shown in phantom line and to the position shown in solid line by the distance X relative to the panel 102. Such “creeping” during the prehemming operation also causes the outer panel to roll upwardly along its outer edge so that the panel 102 begins to bend a position spaced inwardly by the distance Y from the bend line 104. This in turn provides the visual appearance of a relatively wide gap space between the adjacent panels following assembly on the automotive vehicle, as well as distortions like “recoil” that the final hemming operation cannot correct.
With reference to FIG. 3, a second, and perhaps more serious, disadvantage of these previously known hemming methods is that the formation of the flange 100 causes the aluminum panel to become more brittle along the bend line 104 between the flange 100 and the remainder of the outer panel 102. The subsequent final hemming operation causes a further compression of the flange 100 and movement of the flange 100 along its bend line 104. This further compression of the flange and movement along its bend line causes the aluminum panel to crack along the bend line during the hemming operation as shown at 110. Such cracking is unacceptable for the automotive industry as well as other industries.
A still further disadvantage of the relatively large radius used to form the flange with the previously known hemming methods is that the final position of the bend line and thus the outer periphery of the final panel assembly will vary slightly following the hemming operation. Such movement of the bend line of the flange can result from either inward creeping of the bend line or outward compression of the flange bend line during the final hemming operation. Such movement of the outer bend line disadvantageously results in inconsistent gap spacing between adjacent panels on the resulting automotive vehicle.